bug#18545: 24.4.50: Bug - forward-line inside with-selected-window

[lompik@voila.fr]

Hi,

The command `forward-line` fails (no error message) inside a `with-selected-window` statement with 

Here's some steps to reproduce this bug. It is not reproducible with 24.3.

1. emacs -Q
2. In scratch copy these line and C-M-x

(global-set-key (kbd "C-`") '(lambda ()
(interactive)
(with-selected-window (get-buffer-window "*Completions*")
(forward-line 1))))

3. Use these commands:

C-x C-f (select one with lots of file in directory)
C-x C-+ (increase font size)
C-+ 
tab 
tab (open *Completion* window)
C-x o (other-window)
C-x o 
C-x C-+ (increase font size in *Completion* window, you might need more C-+)
C-+ 
C-+
C-+
C-+ 
C-x o (switch back to mnibuffer)
C-h k a (as help while in minibuffer, Important!!)
C-` (forward-line in *Completion* window)
C-` 
C-` 
C-`
C-`
.....(at some point fails to scroll screen

Normal expected behavior is to be able to type C-` until the end of the list in the *Completions* window. With this bug, you will be unable to go past a line that is 
truncated or half displayed. (for example https://cloud.githubusercontent.com/assets/3791334/4373312/be814774-432b-11e4-8add-47167bc1ef9c.png)

Regards !


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